Why are Time Stamps not appearing next to mobile phone icon on Facebook?

[¢нaσѕ™]

I have friends on Facebook Chat (both on my desktop and mobile) that do not have a time stamp next to their mobile phone icon. Some friends will show, for example, "1h" and have a mobile icon next to their name, while others will have that mobile icon but no time stamp. A friends a few hours ago said '6' hours next to mobile icon now it just disappeared What is the explanation behind this? Thank you!

[Jeanne]

If a contact opens the Facebook or Facebook Messenger app on their phone, a "last active" time stamp appears next to their mobile icon in the chat contacts list. others with mobile icon but no time stamp means they open their account in mobile. You dont see the time stamp of others, you can see only your friends timestamps.